Protein phosphatases play a key role in controlling cell growth and proliferation. We have cloned cDNAs encoding four different PTPases and two isoforms of protein serine/threonine phosphatase 2C from mouse testis. The physiological role(s) of intracellular and receptor-linked transmembrane PTPases, their cellular protein substrates, the ligands of receptor-linked R-PTPases, and their functions in tumor suppression will be investigated.